Ancient Clothes Revealed by Forensics

Feb. 28, 2007 — Clothes say a lot about a person. But when it comes to ancient attire, fabrics are often faded, fibers crumbling and designs unrecognizable.

Now, scientists are turning to forensic crime lab techniques to hunt for dyes, paints and patterns in prehistoric textiles.

The technique can not only reveal the beauty of the fabric, but also offers a relatively nondestructive method for analyzing how a piece was constructed, with what fibers and whether the designers used pigments or dyes.

"Forensic photography helps minimize damage by enabling us to sample strategically," said Christel Baldia, assistant professor in textiles at the Florida Institute of Technology in Melbourne.

Baldia worked with colleague Kathryn Jakes, a professor of textile sciences at Ohio State University, to develop the method.

Their approach uses ultraviolet and infrared imagers and filters normally used in crime labs to unveil stains, fingerprints, blood and body oils that cannot be seen in visible light.

This kind of technique works because some molecules absorb light in one wavelength but release it in a different one — sometimes one that the human eye cannot see. The phenomenon is called fluorescence and in forensic photography, this and other optical behavior is captured under infrared or ultraviolet lighting.

It turns out that some pigments (made from minerals) and some dyes (made from plant or animal material) fluoresce. The researchers turned the technique onto fabrics from ancient Native American people known as the Hopewell, who flourished in the Ohio River Valley between 1 A.D. and 500 A.D.

First, the team placed the artifacts under different lighting conditions, including daylight, ultraviolet light and infrared light.

Next, they photographed the fabrics using special film and light-filtering camera equipment. Then they examined the photographs for fluorescence.

The fabrics, drab and colorless in natural lighting, showed changes in colors and patterns under ultraviolet and infrared lighting. The scientists then took very small samples from those regions of the textiles that showed differences and performed conventional tests to determine whether the coloring came from dye or paints.

Conventional sampling requires that a tiny portion of the fabric be immersed in a solution that completely destroys the sample. Using forensic photography can help pinpoint exactly which portion to sample.

"It's good that they are working in this direction, but ultraviolet light is not as nondestructive as it sounds," said Irene Good, an associate of the Peabody Museum at Harvard University and an expert in ancient textiles.

Ultraviolet light induces a chemical reaction in organic materials (think sunburns or fading fabrics in a sunlight window) and could ultimately destroy an entire sample, said Good. Using infrared, which museums already employ to check for forgeries, is preferable, but in the end it's a question of what is more valuable: information or preservation?

"That is what analysts and textile conservators face every day," said Good.